1. Field of the Invention
The present invention is directed to Fe-Cr-Ni-Al ferritic alloys capable of forming a hot oxidation resistive scale of aluminum oxides (chiefly composed of alumina Al.sub.2 O.sub.3) under hot oxidation atmospheres, and more particularly to such Fe-Cr-Ni-Al ferritic alloys combining excellent hot oxidation resistance and improved tensile strength, 0.2% yield strength, elongation, and hardness.
2. Description of the Prior Art
Hot oxidation resistive alloys forming an aluminum oxide scale under hot oxidation atmospheres have been proposed in the art which include Fe-Cr-Al ferritic alloys as disclosed in Japanese Patent Early Publication Nos. 54-141314 and 60-262943, and Fe-Ni-Cr-Al austenitic alloys as disclosed in Japanese Early Patent Publication Nos. 52-78612 and 62-174352. The Fe-Cr-Al ferritic alloys have rather poor mechanical strength nearly equal to ferritic stainless steels and are not expected to remarkably improve the strength even with known heat treatment. Further, in order to form an aluminum oxide (alumina Al.sub.2 O.sub.3) scale of the order of several micrometer (.mu.m) in thickness, the Fe-Cr-Al ferritic alloys should be exposed to high temperature of above 1100.degree. C. for several hours or more. During this heat treatment, the alloy suffers from critical grain growth which reduces the mechanical strength to an unacceptable level for use as a material requiring high mechanical strength. On the other hand, the prior Fe-Ni-Cr-Al austenitic alloys are difficult to provide a uniform alumina (Al.sub. O.sub.3) scale and suffer from a poor scale adherence or flaking of the alumina scale.